1. Field of the Invention
The invention relates to a printed circuit board and a manufacturing method thereof.
2. Description of the Related Art
As a method of manufacturing a printed circuit board, a subtractive method, an additive method, and a semi-additive method are known.
For example, a printed circuit board is manufactured according to the subtractive method as follows (see Japanese Patent Laid-Open No. 2002-252257). To start with, a thin metal film is formed as an adhesion promoting layer (seed layer) by sputtering on a base insulating layer such as a polyimide film. Then, a metal plating layer is formed on the thin metal film.
The metal plating layer is then coated with photoresist in a prescribed pattern, followed by drying, and then subjected to process of exposure, development, and etching. Then, the photoresist is removed and a complete conductive (interconnection) pattern forms.
In another example, a printed circuit board is manufactured according to the semi-additive method as follows (see Japanese Patent Laid-Open No. 2002-368393). To start with, a thin copper film is formed on a base insulating layer by sputtering. Then, a plating resist pattern is formed to expose a predetermined interconnection part of the thin copper film. Then, the exposed part of the thin copper film is provided with electrolytic nickel plating, and then the plating resist pattern and the thin copper film in the region of the plating resist pattern are removed. In this way, a complete conductive pattern forms.
In some cases, when the thus produced printed circuit board is used in a high temperature and high humidity atmosphere, ion migration of the conductor (copper in the above example) is caused along the boundary between the base insulating layer and a cover insulating layer formed on a surface of the base insulating layer having the conductive pattern thereon. This short-circuits conductive patterns formed as interconnections.
Herein, the ion migration refers to the movement of metal ions (copper ions in the above example) through water as a medium while an electric field is generated.